Electromagnetic heating unit



ELECTROMAGNETIC HEATING UNIT Andrew Alford, Winchester, Mass. (299 Atlantic Ave., Boston, Mass.)

Filed Aug. 19, 1957, Ser. No. 678,777

16 Claims. (Cl. 219-1055) The present invention relates to a heat producing means for heating electrically conductive or partially conductive objects by inducing currents in them. More particularly, the present invention relates to a furnace or oven adapted for use in treating various objects such as melting metal, heating metallic objects to soldering temperatures, as well as cooking various semi-conductive foods.

In the present status of the art of heating by electromagnetically induced currents of high frequencies, a substantial problem has arisen with respect to the variable load impedances presented to the ultra-high frequency oscillators which are used. As such oscillators are not well adapted to operate into loads which vary over wide impedance ranges, it is quite desirable to provide means by which the impedance presented to the oscillators wiil remain substantially constant, regardless of the specific object being heated by induced currents.

The present invention is designed to overcome this problem through the utilization of a circuit arrangement which includes a radio frequency network, hereinafter termed a hybrid, and of the type disclosed in pending patent application Serial No. 550,242, fied November 23, 1955, by Chester B. Watts, Jr. Such hybrids are also described in the publication Transmission Line Hybrids, published by Alford Manufacturing Co., Inc., of 299 Atlantic Avenue, Boston, Massachusetts. (Copyright, 1955.) US. Patent No. 2,769,146, issued October 30, 1956.

In the arrangement of the present invention, a pair of radiating elements are connected to opposite terminals of the hybrid, with the ultra-high frequency oscillator connected to the input and a terminating load connected to the terminal opposite the input terminal. By properly selecting the lengths of the transmission lines between the radiating elements and hybrid, as well as providing means for balancing out the reactance of both radiating elements, not only can the impedance presented to the oscillator be maintained within a relatively narrow range, but also it is possible to determine proper adjustments of the radiating element impedances for maximum efliciency.

These and other objects of the present invention will be more clearly understood when considered in conjunction with the accompanying drawings, in which:

Figure 1 is a schematic diagram of a circuit used to explain the purpose of the present invention.

Figure 2 is a schematic diagram of a hybrid adapted for use with the present invention.

Figure 3 is a schematic circuit diagram of one form of the present invention.

Figure 4 is a schematic diagram of a second form of the present invention.

Figure 5 shows a variation of the invention in which orthogonally oriented dipoles heat the load.

Consider an arrangement comprising a very high frequency oscillator 1 which is used to energize a coil 2 through transmission line 3, as shown in Figure 1. semi-conductive object 4 is placed into the electro-magnetic field produced by coil 2. The heating in the semi- States Patent Other types of hybrids are disclosed in conductive object is produced by the flow of currents that are induced by the electro-magnetic field. These currents produce an electro-magnetic field of their own, and this secondary electro-magnetic field, by inducing a countervoltage in coil 2, results in a load impedance being presented at the end of transmission line 3. This induced load impedance will obviously vary with the size of the semi-conductive object, with the shape of this object, the conductivity of this object and the orientation, as well as the position of the object with respect to the coil. If such apparatus is used, for ordinary cooking, a variety of different semi-conductive objects will be placed in the field of the coil, and consequently a great variety of different impedances will be presented to the end of the transmission line 3, and, therefore, also to oscillator 1.. Since ultra-high frequency oscillators are not very well adapted to operate into loads of widely varying impedances, an arrangement, particularly shown schematically in this figure is not well suited for use in electro-magnetic furnaces and ovens.

Solutions of the problem thus presented are suggested by the arrangement schematically illustrated in Figures 3 and 4. Prior to a consideration of these particular arrangements, an understanding of the hybrid herein referred to is necessary. This hybrid schematically illustrated in Figure 2, may be fully understood from a consideration of the above mentioned co-pending application, or the publication to which a reference has been made. For convenience in understanding the structure and operation of a typical hybrid, reference is made to Figure 2. As illustrated, the hybrid consists preferably, in one form, of a U-shaped coaxial transmission line assembly 50 mounted in a carefully made cavity 51 of rectangular cross section. The intermediate outer conductor is interrupted by a small gap MN with the inner conductor 52 passing through this gap and itself having a U shape. Four terminals, for convenience, labelled I, II, S and P are provided as illustrated. The outer conductors of each of these terminals is continuous with the cavity 51, while the inner conductors of terminals I and II are continuous with the intermediate outer conductor 50. The inner conductor or terminal S is continuous with inner conductor 52 and the inner conductor of terminal P is continuous with the outer conductor 50. Opposite potentials are developed across the gap M--N. Under such conditions, equal but opposite potentials are applied to loads connected to terminals I and II. Equal waves of opposite potentials traveling from M and N towards terminal P, cancel each other at this terminal, thus substantially no power is delivered to the load connected to terminal P.

If it is assumed that a generator is connected to terminal S and equal loads were connected to terminals I and II, whether capacitive or inductive, regardless of whether they matched the load at terminal P, the waves traveling along the U shaped conductor towards the terminal P, would cancel each other. Under such conditions, the isolation hybrid may be considered to be acting as a balun.

If however, two inequal loads are connected to terminals I and II, then the two waves traveling along each leg of the U, differ from each other either in magnitude or in phase, or both, and therefore no longer cancel at terminal P. In fact the magnitude of the resulting current at P may be used as a measure of the inequality of the loads connected to terminals I and II in a manner which is considered in the above mentioned co-pending application. Further, if power is introduced, either by a second enerator or bv an induced counter-voltage at terminal P. the potentials developed across equal loads connected to terminals I and II have the same relative phase and under such conditions there is no difference of potential across gap MN, due to such power. Under these circumstances there is no power delivered to terminal S.

Utilizing a hybrid as thus considered, a satisfactory arrangement for exciting heat producing radiating elements is illustrated in Figure 3. In this arrangement, instead of a single radiating element, such as a coil, a pair are utilized, which in the specific construction herein discussed, comprise a pair of coils 21 and 22. These coils are arranged at right angles to each other, in such a manner that they do not induce current into one another. As illustrated this may comprise arranging a pair of square shaped coils with coincident center vertical axes in planes at right angles to one another, with one coil being positioned, just above the other and the two coils interlocking. These coils should be arranged with suitable reflectors, such as formed by a metal enclosing box 28 shown with portions cut away in Fig. 3 to expose the coils. These coils are respectively connected by transmission lines 23 and 24 to a hybrid 25 with transmission line 23 connected to terminal 11 and transmission line 24 connected to terminal I. The hybrid which may be used is preferably Type 1024, as described in United States Patent No. 2,769,146 by the Alford Manufacturing Company of 299 Atlantic Avenue, Boston, Massachusetts. An ultra-high frequency oscillator 26 having a suitable power supply (not shown) is connected to the P terminal of the hybrid 25. A resistive load 27, suitably matched is connected to the S terminal of the hybrid 25. Transmission line 24 is made longer by preferably a quarter of a wave length of the frequency delivered by the oscillator than line 23. If Z equals the impedance looking into the line 23 and Z the impedance looking into the line 24, the product Z Z =Z where Z is a characteristic impedance of the transmission lines 23 and 24. By thus properly selecting the relative lengths of transmission lines 23 and 24, the impedances Z and Z are almost purely reactive and are inverse to one another, when there are no conductive or semi-conductive objects in the field of two coils 21 and 22. Under such circumstances the power from the oscillator passes through the hybrid 25 into the terminating load 27. The impedance looking into the P terminal of the hybrid is then substantially a pure resistance equal to that of the characteristic im-. pedance of the hybrid. If a semi-conductive object is now introduced into the field of the coils 21 and 22, as indicated at 20, the impedances Z and Z are no longer purely reactive, but become partially resistive and the power which had been traveling almost substantially into the load 27 is now partially diverted into impedances Z and 2;. When Z and Z become equal to each other, no power is delivered into the load 27, but all the power goes into the heating of this semi-conductive object through the coils 21 and 22. It is noted that all through the changes from the very small semi-conductive object in the field of coils to a large object which results in the transmission lines 23 and 24 becoming better matched, the impedance seen looking into the P-branch of the hybrid 25, remains substantially close to the characteristic impedance of the hybrid so that the oscillator is properly loaded and can function at high efficiency.

While the arrangement thus shown in Figure 3 is satisfactory, it is further desirable to provide means for balancing out the reactance of both coils. Such an arrangement is shown in Figure 4, which is similar to that shown in Figure 3, except for such means for balancing out the reactance of both coils. In this arrangement, equal variable capacitors 31 and 32 are connected respectively in series with coils 21 and 22. These capacitors 31 and 32 are provided with a common operating shaft 33, which is utilized to effect the reactance of both coils equally and simultaneously. Thus by turning the shaft 33, the impedances Z and Z respectively presented by the transmission lines 23 and 24, become either more equal to .each other or less equal to each other, dependent upon the direction of rotation of the shaft 33. If

d the impedances of transmission lines 23 and 24 become more nearly equal to each other, they also become more resistive.

As indicated above, when the transmission line 23 and 24 become more resistive and therefore less inverse to one another, the power to the load 27 is accordingly reduced. The measure of this reduction is a measure of the efficiency of power transfer from the oscillator to the semi-conductive materials. By providing a crystal rectifier 34 in series with a D.C. milliammeter 36 in turn in parallel with the by-pass condenser 35, a suitable reading on the milliammeter 36 may be obtained in order to determine the proper direction in which to turn the shaft 33. The crystal 34 is energized through a condenser 37' connected to the inner conductor of the coaxial line between the S terminal of the hybrid and the resistor 27.

It will be clear to those versed in the art that the two condensers 31 and 32 illustrated in Figure 4 are merely representative of a number of means comprising for example, groups of variable reactances or adjustable coupling means that could be employed to achieve a better match of the impedance introduced into the heating coils to the characteristic impedance of the transmission lines. The most convenient arrangement is one in which a single control is used to vary impedance match between lines 23 and 24 and coils 21 and 22, in an equal manner so that the product of the impedances Z and Z will remain equal to Z for when this is the case, the impedance seen by the oscillator looking into the hybrid then remains close to a fixed value Z If the impedance match between lines 23 and 24 and coils 21 and 22 were carried out independently of each other, it would be possible to make impedances Z and Z equal to each other without each being equal to Z Under such circumstances the load 27 would receive no power from the oscillator and therefore no measure on the milliammeter could readily be obtained. The impedance looking at the terminal P of the hybrid 25, would not be matched. It is therefore desirable to adjust the impedances of the two coils equally, in order to be assured by visible means that a greater portion of the total power is diverted into the heating of the object while the impedance looking into the hybrid from the oscillator remains approximately matched.

Under some conditions, when for example, a long thin object is placed parallel to one coil, but perpendicular to the other one, unequal values of resistance will be introduced to the two coils and impedances Z and Z will not be equal unless equalized by independent matching. Under such conditions, in general, a substantial amount of power will be delivered to the load 27 indicating that the object is not placed correctly in the field of the two coils and therefore should be turned in another direction for a more efiicient operation.

While the foregoing invention has been described with the use of coils as radiating elements, it should be understood that these coils are convenient devices for producing the desired field at low frequency, as for example, frequencies at 50 megacycles or lower. When operating at ultra high frequencies, other means, as for example, a pair of dipole antennas 29 and 30 arranged at right angles to each other and each placed at a quarter of a wave length from opposite sides of metal-lined box 28 as shown in Fig. 5, could be utilized for effective production of the desired electromagnetic field. When such antennas are used, they may be energized with transmission lines connected to the hybrid in the same manner as coils are connected to the hybrid as described in connection with Figures 3 and 4.

When utilizing antennas it is also desirable to properly adjust the impedances of the antennas. Such means for proper adjustment may take a variety of forms, which include for example, movable metal vanes in the neighborhood of the antennas or variable shunt reactances or combinations thereof.

Having now described my invention, I claim:

1. A high frequency inductive heating device for heating a dissipative load comprising a pair of electromagnetic radiating devices of equal impedances oriented with respect to one another to radiate substantially coincident electromagnetic fields normal to one another in a region bounded by both said devices, said devices defining a volume internally thereof for accommodating said dissipative load, a high frequency oscillator, a terminating load, a hybrid junction having four branches adapted to receive energy on one branch and transmit the same to two other branches when said other branches are terminated in equal impedances, with one said branch and the remaining branch isolated from each other with respect to energy introduced through said one branch, means connecting said devices one each to said other branches, means connecting said load to said remaining branch and said oscillator to said one branch.

2. A high frequency inductive heating device for heating a dissipative load comprising a pair of electromagnetic radiating devices of equal impedances oriented with respect to one another to radiate substantially coincident electromagnetic fields normal to one another in a region bounded by both said devices, said devices defining a volume internally thereof for accommodating said dissipative load, a high frequency oscillator, power transmission means coupling said devices to said oscillator and means including a hybrid junction interconnected to said last mentioned means adapted to transmit power to said devices and present a substantially constant impedance to said oscillator.

3 Means for heating semi-conductive and conductive objects by electromagnetically induced currents comprising a pair of electromagnetically radiating devices oriented with respect to one another to radiate substantially coincident electromagnetic fields normal to one another in a region bounded by both said devices, said devices defining a volume internally thereof for accommodating said objects, a high frequency oscillator, a terminating load, a hybrid junction having four terminals and adapted to receive energy in one terminal and transmit the same to two other terminals when said other terminals are terminated in equal impedances with said one terminal and the remaining terminal isolated from each other with respect to energy introduced through said remaining terminal, means connecting said devices one each to said other terminals, means connecting said load to said remaining terminal and said oscillator to said one terminal.

4. A device as set forth in claim 3, wherein the power appearing at said terminal load is a measure of the impedance mismatch between said other terminals and having means for measuring said power.

5. Means for heating semi-conductive and conductive objects by electromagnetically induced currents comprising a pair of electromagnetically radiating devices oriented with respect to one another to radiate substantially coincident electromagnetic fields normal to one another in a region bounded by both said devices, said devices defining a volume internally thereof for accommodating said objects, a high frequency oscillator, a terminating load, a hybrid junction having four terminals and adapted to receive energy in one terminal and transmit the same to two other terminals when said other terminals are terminated in equal impedances with said one terminal and the remaining terminal isolated from each other with respect to energy introduced through said remaining terminal, a pair of transmission line means each interconnecting a different one of said other terminals to a different one of said devices, means connecting said load to said remaining terminal, and means connecting said oscillator to said one terminal.

6. Means for heating semi-conductive and conductive objects by electromagnetically induced currents comprising a pair of electromagnetically radiating devices oriented with respect to one another to radiate substantially coincident electromagnetic fields normal to one another in a region bounded by both said devices, said devices defining a volume internally thereof for accommodating said objects, a high frequency oscillator, a terminating load, a hybrid junction having four terminals and adapted to receive energy on one terminal and transmit the same to two other terminals when said other terminals are terminated in equal impedances with said one terminal and the remaining terminal isolated from each other with respect to energy introduced through said remaining terminal, a pair of transmission line means each interconnecting a different one of said other terminals to a different one of said devices, said transmission line means differing in length by a quarter wavelength of the frequency of said oscillator, means connecting said load to said remaining terminal and means connecting said oscillator to said one terminal.

7. Means as set forth in claim 6, wherein said devices each comprise a coil with the coils arranged at right angles to one another.

8. Means as set forth in claim 6, wherein said devices each comprise a coil having four sides forming a square with the planes of said coils normal to one another and the coils interlocked.

9. A means as set forth in claim 6, wherein said devices comprise dipole antennas positioned at right angles to one another and enclosed within a metal box, said antennas being spaced a quarter of a wavelength of the operating frequency of said oscillators from the opposite walls of said box.

10. Means as set forth in claim 6, wherein means are provided for balancing the reactances of said devices.

11. Means as set forth in claim 10, wherein means are provided for measuring the degree of balance of said devices.

12. Means as set forth in claim 6, wherein means are provided for each device for balancing the reactance of said devices, and single control means are provided for simultaneously operating said last mentioned means.

13. Means as set forth in claim 6, wherein variable impedance matching means are provided for each device, and single control means are provided for simultaneously varying said last mentioned matching means whereby the reactances in each line may be balanced, thereby presenting substantially equal resistive loads at said other terminals.

14. A device as set forth in claim 13, wherein said impedance matching means each comprise a capacitor and said single control means includes a common operating shaft for said capacitors.

15. Apparatus for electromagnetically heating a dissipative load comprising, a source of high frequency en ergy, a hybrid junction having a pair of side terminals, a series feed terminal and a shunt feed terminal, means for applying said high frequency energy to said shunt feed terminal, a terminating resistance connected to said series feed terminal, first and second radiating elements, first and second wave transmission conduits respectively coupling said radiating elements to respective ones of said side terminals, and conducting reflectors for substantially confining energy radiated by said elements into a volume internally thereof closely adjacent to said radiating elements, said volume being arranged to accommodate said load.

16. Apparatus for electromagnetically heating a dissipative load comprising, a source of high frequency energy, a hybrid junction having a pair of side terminals, a series feed terminal and a shunt feed terminal, means for applying said high frequency energy to one of said series feed and said shunt feed terminals, a terminating resistance connected to the other of said series feed and said shunt feed terminals, first and second radiating elements, first and second wave transmission conduits respectively coupling said radiating elements to respecthe ones of said side terminals, and conducting reflectors for substantially confining energy radiated by said elements into a volume internally thereof closely adjacent to said radiating elements, said volume being arranged to accommodate said load.

References Cited in the file of this patent UNITED STATES PATENTS Clough Apr. 2, 

